Drive ST head stays still with small clicks

[Roberto]

Hello everyone!
Today I have a day off and when I have it I want to mess with the hardware that has been there for years and it doesn't work.
This morning I recovered from the pile this Floppy Drive for Atari ST, internal, double head, and when I put the floppy you can see that it is trying to light but the head goes a little jerky and stays there. If I try to move the head manually, you can see that it then returns exactly to the initial position, thus continuing with these snap movements.
I tried to clean both the two heads and the inside of the sensor (the one where there is the iron slat that moves to be clear).
Is that the sensor of track 0?
Can any of you advise me what I could do?
In the Desktop I see correctly the two icons of the floppy drive.

Here is the video:

[stephen_usher]

Looks like it can't read track 0

[Roberto]

Looks like it can't read track 0

Could it be this mechanism that determines track 0? It is a spring-loaded pin that is pressed by the same floppy disk inserted. This pin lifts a strange rectangular thing (see photo) placed on the other side just to get closer to a kind of "long light bulb" (see photo) placed in the card above.

IMG_6166 - dimensioni grandi.jpeg

IMG_6165 - dimensioni grandi.jpeg

IMG_6168 - dimensioni grandi.jpeg

[stephen_usher]

It's not that it couldn't FIND track zero, it just couldn't read the track data off the disk. It could obviously successfully seek there correctly.

[PhilC]

Just a thought, is it actually detecting it has a disk inserted? Might be worth checking the micro switches for that.

[Darklord]

Assuming you've tried different floppy disks and checked/examined
the floppy drive cable...

[rubber_jonnie]

This pin lifts a strange rectangular thing (see photo) placed on the other side just to get closer to a kind of "long light bulb" (see photo) placed in the card above.

That 'long light bulb' you're talking about is a reed switch and the item that you're describing as a 'strange rectangular thing' is likely a magnet. As it moves closer to the reed switch, the magnet will operate the switch and is probably what tells the drive a disk is inserted.

However, as @stephen_usher has stated, it looks like it can't read the contents of track 0 rather than not being able to find track 0, which is often determined by some sort of optical switch.

In fact this appears to be the track 0 sensor to the left of the heads:

Screenshot 2025-10-31 at 09.20.33.png

The arm rotates and operates the fixed optical sensor when it rotates to about the 7 to 8 o'clock position where the end of the arm is inside the black plastic piece. If you operate the heads manually you'll see it rotate.

Have you tried to format a blank disk in the drive to see how it behaves?

[Roberto]

.....
The arm rotates and operates the fixed optical sensor when it rotates to about the 7 to 8 o'clock position where the end of the arm is inside the black plastic piece. If you operate the heads manually you'll see it rotate.

Have you tried to format a blank disk in the drive to see how it behaves?

I state that from my pile of hardware, left there a long time ago, I recovered two internal units that have the exact same problem.
I also recovered an external unit (SF314) that has the same problem with the latter.
All these units have a common denominator, which is that they try to read track 0 with that classic tick tick from the video I posted at the beginning.

I would say very strange that three units taken so randomly, have the same problem....

@rubber_jonnie
Yes, I tried to format more DD floppy disks (the specific one since HD floppy disks can give problems on DD drives), but nothing.
Consider that I have several Atari and I know that the floppys are intact and working without any problem.

I also answer to @PhilC:
I think the drives (at least the internal ones), are NOT detecting the inserted disk.

-@Darklord arklord
Yes, I checked the Flat cable on all three drives and tested them on different Atari ST. I also have other floppy drives that work regularly on my Atari where I ran the tests.

Things I'm sure of (it applies to all three Floppy Drives):
- The heads move freely on the tracks (The diagnostic cartridge confirmed it to me)
- The heads are clean (isopropyl alcohol used)
- The problem is not on the Atari machine (I used 3 different ones that work correctly with their drive)

Do I have a doubt? Is it possible that it is the fault of this capacitor or are there rubber belts in these units?

Update: I just found out that the drives (at least the internal ones), are not detecting the inserted disk.
Update 2: I tried to weld two cables at the ends of the glass bulb (the one that determines whether the floppy is inserted or not), and I saw that, measuring the values with a tester, when the floppy is inserted it measures 1.7 Ohm, while when the floppy is not there, it measures 5.4 ohms. I only took this measurement on one of the 3 units.

My units:

IMG_6193 - dimensioni grandi.jpeg

IMG_6192 - dimensioni grandi.jpeg

[Darklord]

Okay, just making sure I've got this straight.

These 3 drives work correctly on a different ST.

The ST you're testing does work with floppy drives other then these 3
drives.

If so, that's a little odd... :)

[Roberto]

Okay, just making sure I've got this straight.

These 3 drives work correctly on a different ST.

The ST you're testing does work with floppy drives other then these 3
drives.

If so, that's a little odd... :)

Hi Darklord, no, these three units do NOT work on all my Atari.
My Atari do work with their original floppy drives.